THE OBJECTIVE OF THIS PROPOSAL IS TO MODEL THE REGIONAL-SCALE STRUCTURE IN MERCURY S INTERNAL AND EXTERNAL MAGNETIC FIELDS. FROM MAGNETOMETER (MAG) OBSERVATIONS ACQUIRED DURING FLYBYS OF THE MERCURY SURFACE SPACE ENVIRONMENT GEOCHEMISTRY AND RANGING (MESSENGER) SPACECRAFT BY MERCURY WE CONFIRMED THE DOMINANTLY DIPOLAR NATURE AND CORE DYNAMO ORIGIN OF THE INTERNAL FIELD. USING ORBITAL DATA WE HAVE ESTABLISHED AN OFFSET AXIAL DIPOLE (OAD) DESCRIPTION FOR THE CORE FIELD GEOMETRY THAT DESCRIBES THE STRENGTH AXISYMMETRY AND NORTHWARD OFFSET OF THE MAGNETIC EQUATOR FROM THE PLANETARY EQUATOR. UNTIL NOW IT HAS NOT BEEN POSSIBLE TO EXPLOIT MESSENGER OBSERVATIONS TO FULLY CAPTURE THE CORE FIELD MORPHOLOGY BECAUSE OF THE NEED TO ACCOUNT FOR THE LARGE CONTRIBUTIONS OF EXTERNAL FIELDS (UP TO 40% OF THE MEASURED FIELD). ANY ADVANCE IN RESOLVING STRUCTURE IN THE INTERNAL FIELD BEYOND THAT DESCRIBED BY THE OFFSET AXIAL DIPOLE (OAD) DEPENDS ON REMOVING THESE EXTERNAL SIGNALS BUT COULD PROVIDE KEY DISCRIMINANTS AMONG DIFFERENT DYNAMO MODELS FOR MERCURY. THE TIME-AVERAGED EXTERNAL FIELD HAS BEEN MODELED AS THE COMBINATION OF THE OAD AND MAGNETOTAIL CURRENT SYSTEMS CONFINED WITHIN A REALISTIC MAGNETOPAUSE [KORTH ET AL. 2015]. THIS MODEL DOES HOWEVER NOT ACCOUNT FOR VARIATIONS IN MAGNETIC ACTIVITY OR CONTRIBUTIONS FROM BIRKELAND CURRENTS WHICH HAVE SIGNALS OF ~50 NT AND EXTEND DOWN TO THE PLANETARY SURFACE. BOTH MUST BE CONSIDERED IN DETERMINING MERCURY S INTERNAL FIELD BECAUSE THEIR CONTRIBUTIONS ARE LARGE OVERLAP IN THEIR SPATIAL POWER SPECTRUM WITH POSSIBLE CORE FIELD SIGNATURES CAN BE ALIASED INTO INTERNAL FIELD SIGNATURES BECAUSE OF MESSENGER S ORBIT GEOMETRY AND IN THE CASE OF FIELDS DUE TO BIRKELAND CURRENTS INCREASE WITH DECREASING ALTITUDE. TO ADVANCE THE ANALYSES OF MERCURY S MAGNETIC FIELD WE PROPOSE TO DEVELOP A COMPREHENSIVE MAGNETOSPHERIC MODEL FOR MERCURY PRODUCE A DATASET OF MAGNETIC FIELD RESIDUALS RELATIVE TO THIS MODEL AND DETERMINE ANY STRUCTURE IN MERCURYS CORE FIELD BEYOND THAT DESCRIBED BY THE OAD. THIS INCLUDES THE FOLLOWING: (1) ESTABLISH DEPENDENCES OF THE EXTERNAL FIELDS ON MAGNETIC ACTIVITY AND INCORPORATE THESE AND THE SIGNATURES OF BIRKELAND CURRENTS INTO A NEW MAGNETOSPHERIC MODEL; (2) PRODUCE VALIDATE AND ARCHIVE AT THE PDS PDS4-CONFORMANT DATA PRODUCTS OF MESSENGER MAGNETIC FIELD RESIDUALS RELATIVE TO THE MAGNETOSPHERIC MODEL FOR THE INTERVAL FROM ORBIT INSERTION (23 MARCH 2011) THROUGH END OF ORBITAL OPERATIONS (30 APRIL 2015); AND (3) USE THE NEW RESIDUALS DATASET TO CHARACTERIZE REMAINING STRUCTURE IN MERCURY S INTERNAL FIELD USING TECHNIQUES IN INVERSE THEORY TO MAXIMIZE THE RECOVERY OF SIGNATURES IN THE NORTHERN HEMISPHERE WHERE MESSENGER DATA COVERAGE WAS BEST AND SPACECRAFT ALTITUDES WERE LOWEST. THE RESULTS FROM OUR STUDY ARE CRITICAL TO ADVANCE OUR UNDERSTANDING OF THE INTERACTIONS BETWEEN MERCURY S INTERIOR SURFACE AND MAGNETOSPHERE TO CONTRIBUTE TO THE DETERMINATION OF HOW THE PROCESSES THAT SHAPE PLANETARY BODIES OPERATE AND INTERACT AND ULTIMATELY TO ANSWER QUESTION 2 OF THE SOLAR SYSTEM ROADMAP 2006: HOW DID THE SOLAR SYSTEM EVOLVE TO ITS CURRENT DIVERSE STATE? THE SOLAR SYSTEM ROADMAP EMBODIES THE STRATEGIC GOALS FOR SOLAR SYSTEM EXPLORATION SO THAT THE PRODUCING VALIDATING AND ARCHIVING THE PROPOSED DATA PRODUCTS IS DIRECTLY RELEVANT TO THE PLANETARY SCIENCE RESEARCH PROGRAM AND IN SPECIFIC TO THE DISCOVERY DATA ANALYSIS PROGRAM (DDAP).